1. Field of the Invention
The present invention is broadly concerned with improved isoparaffin-olefin alkylation processes used for the conversion of low carbon number paraffins and olefins into valuable, high octane number compounds useful in gasolines. More particularly, the invention pertains to such alkylation processes wherein the problem of coke laydown and catalyst deactivation is minimized and the production of high octane number alkylates is enhanced by carrying out the alkylation reaction at near-critical or supercritical conditions and by using a co-solvent or diluent such as carbon dioxide in a molar excess.
2. Description of the Prior Art
Alkylation processes are used industrially to convert light refinery gases such as C.sub.4 -C.sub.5 isoparaffins into more valuable branched chain gasoline-range C.sub.7 -C.sub.9 alkylate compounds. Particularly valuable alkylates are trimethylpentanes (TMPs) and 2,2-dimethylbutane (neohexane) which are used as high-octane blending components for aviation and civilian gasolines. It is estimated that about 13% of the U.S. gasoline pool is made up of alkylates.
Conventional alkylation processes utilize HF or H.sub.2 SO.sub.4. These acidic processes are undesirable for a number of reasons including environmental and transportation hazards, and the difficulty of economical acid disposal or regeneration. Alkylation processes are also known where the isoparaffin and olefin reactants contact a zeolite, sulfated zirconia, wide-pore MCM-type or other solid alkylation catalysts at elevated temperature and pressure reaction conditions.
A significant problem with solid-catalyzed alkylation processes is the tendency for the catalysts to rapidly deactivate by virtue of coke laydown on the catalyst which tends to plug the catalyst pores. This problem is compounded in that most solid alkylation catalysts cannot be regenerated without causing irreversible degradation of the catalysts. Certain reactor operating strategies have been proposed to mitigate the problem of coke laydown, such as the use of a slurry reactor or supercritical operation. However, these efforts have not been truly successful; for example, at the high temperatures required to achieve supercritical reaction conditions when not using a co-solvent or diluent such as carbon dioxide, undesirable side reactions such as oligomerization and cracking are favored. As a consequence, while catalytic alkylation processes have shown promise, a number of intractable problems exist which limit the practical utility thereof.